A current scheme for making changes to electronic substrates, such as ceramic, glass or glass-ceramic substrates, uses redundant connection pads to add new connections or delete existing connections. The original connections from the integrated circuit chips, hereinafter referred to as IC's, to the electronic substrate are normally made using solder or braze connections between the IC's and the connection pads located beneath the IC's. Surplus connection pads, each of which is connected to an original connection pad, are located around the periphery of the IC's. New or additional connections may be made between redundant connection pads, for example, by wire bonding, staple bonding or by the use of decals. Connections may be deleted by severing the line connecting a redundant pad to an original pad. If, during electronic substrate rework, an IC must be removed and replaced, solder and braze removal is limited to the area that was directly below the IC, leaving the peripheral connections undisturbed.
However, because space on a substrate is both scarce and valuable, it is desirable to use the available space between the original connection pads beneath the chip itself, rather than, or in addition to, the peripheral area, to make such circuit changes. In such a scheme it becomes necessary to selectively remove solder or braze from the original connection pads without disturbing the immediately adjacent areas where fragile connections, such as wire bonds, staple bonds or decals, may be present. Additionally, if the redundant connection pads are deposited with a layer of gold, it may be desirable to prevent solder or braze from depositing over the gold or dissolving the gold during rework.
The ability to selectively remove solder or braze from a surface is similarly desirable for making changes to any electronic circuit substrate which uses surface-mounted IC's, components or wiring, for example, epoxy-based electronic cards.
A variety of devices for removal of solder or braze from electronic substrates have been previously disclosed. The disclosed devices can be classified by two approximate categories, the first being flat-surface blocks for the solder or braze removal, and the second being wicks for the removal of solder or braze from the surface of an electronic substrate.
Within the flat-surface block category, for example, is disclosed a tinned copper slug for module reworking, "Use of a Tinned Copper Slug for Module Reworking", IBM Technical Disclosure Bulletin, Vol. 24, No. 7A, p. 3481 (December 1981). A tinned copper slug is applied to a substrate and the tinned copper slug absorbs melted solder by capillary action as the substrate is heated.
Also disclosed in the flat-surface block category is a porous copper block having solid plated studs, "Chip Rework on Multilayer Ceramic Recess", IBM Technical Disclosure Bulletin, Vol. 27, No. 10B, p. 6344 (March 1985). A porous copper block having solid plated studs is placed on a substrate, such that the solid plated studs enter substrate recesses where excess solder is located. As the copper block is pushed down, the solid plated studs are forced into the recesses, the excess solder is then pushed outwardly and it is then absorbed by the base portion of the porous copper block.
Several variations of wicks capable of absorbing solder by capillary action have been disclosed. For example, U.S. Pat. No. 4,164,606 discloses a wick formed by braiding strands of copper that have been individually coated with solder. U.S. Pat. No. 4,323,631 discloses a wick formed from strands of non-metallic material which are coated with metal and flux to render the fibers more capable of wetting. U.S. Pat. No. 4,416,408 discloses a wick comprised of an open mesh of strands.
Several methods for making porous metal devices have been previously described. Disclosed in IBM Technical Disclosure Bulletin, Vol. 25, No. 5, p. 2285 (October 1982), is a method for making porous metal devices which employs a single sintering step. A mold is filled with copper powder, vibrated, covered and heated to a temperature of 890 degrees Celsius in an atmosphere of disassociated ammonia. After cooling, the copper devices, which have undergone approximately 10 percent shrinkage, are removed from the mold. U.K. Patent No. 661,780 discloses a strikingly similar process for making porous metal molds to be used to make pottery and wood fiber egg trays.
Australian Patent No. 162,811 also discloses a method for making porous metal devices. Spherical particles are used which have a metal coating that is softer than the core metal and preferably alloyable with the core metal. The particles are compressed into a briquette, such that the softer coating is distorted and holds adjacent particles together. The briquette is then sintered such that the softer coating alloys with the harder core.
U.S. Pat. No. 2,219,423 discloses a method for forming porous metal articles of a complex shape. Flat briquettes are placed over a mold having a complex shape. The mold cover which may have a shape that matches the mold is placed over the briquette before sintering. During sintering the weight of the mold cover presses the briquette until it conforms to the shape of the mold.